1. Field of the Invention
The present invention relates to an image sensing device used in a digital camera or a cellular phone with a camera and, more particularly, to an image sensing device which uses a prism having a free-form surface as a reflecting surface, an image sensing apparatus using the image sensing device, and an image sensing position switching method.
2. Description of the Related Art
In recent years, a number of applications for image sensing apparatuses which use a coaxial optical system have been filed as image sensing apparatuses used in digital cameras or cellular phones with a camera. In a coaxial optical system, optical elements such as a lens are rotationally symmetrical with respect to the optical axis (an axis which connects the center of the aperture of the image sensing system and the center of the image sensing screen) of the optical system. Image sensing apparatuses having a coaxial system are disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publications No. 2001-272587 (reference 1), No. 2002-267928 (reference 2), and No. 2002-320122 (reference 3).
Recent digital cameras and cellular phones with a camera are required to be compact and thin and have high performance. In these devices, if the image sensing device using a coaxial optical system should be compact, the number of lenses must be decreased. However, when the number of lenses is decreased, aberrations generated in the optical system can hardly be suppressed, resulting in poor image quality. To obtain a high image quality, the number of lenses must be increased. As a result, the image sensing device becomes bulky.
As a means for solving these problems, image sensing apparatuses using an eccentric optical system have been proposed. Image sensing apparatuses using an image sensing optical system using a prism with a free-form surface are disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publications No. 11-326766 (reference 4), No. 2002-196243 (reference 5), and No. 2003-84200 (reference 6).
In this specification, a “free-form surface” means a curved surface which is rotationally asymmetrical with respect to the optical axis of the light beam which strikes the surface or the optical axis of the light beam which exits from the surface and has only one mirror image surface along these optical axes.
The techniques described in references 4 to 6 aim at obtaining a compact device and a high-quality image by forming an image sensing optical system by using a prism having a free-form surface as a light incident surface, light exit surface, or reflecting surface. Especially, in references 5 and 6, two prisms are combined. The light incident surface, reflecting surface, and light exit surface of the first prism close to the object and the light incident surface, two reflecting surfaces, and light exit surface of the second prism close to the image sensing surface, i.e., a total of seven surfaces are formed as free-form surfaces.
The characteristic features of such an optical system are as follows.
(1) The three reflecting surfaces are formed from free-form surfaces having a power (refracting power). These reflecting surfaces can obtain a large power and are rarely affected chromatic aberration as compared to a refractive optical system such as a lens.
(2) The seven optical surfaces can be formed in a compact space. Hence, the optical elements are concentratedly set in the limited space.
(3) To obtain high optical performance, the optical path length of the entire optical system is preferably long to some extent. When the optical path is bent by using such a prism optical system, the optical path length of the optical system can be long, and the entire size can be small.
For these reasons, a high image quality can be obtained by a compact device.
The optical system described in Jpn. Pat. Appln. KOKAI Publication No. 7-333505 (reference 7) includes a reflecting mirror, an optical system by a lens, and a reflecting mirror sequentially from the object side. As compared to this system, the optical system described in reference 5 or 6 can reduce the width. For this reason, a more compact image sensing device can be provided.
When an image sensing device is used in a compact and thin digital camera or a cellular phone with a camera, the angle of view of the image sensing optical system may be set to a wide angle to implement pan focus. In pan focus, an in-focus state is obtained almost throughout the object distance range by fixed focusing.
Recently, demands for reading a barcode or letters on books or originals by using a digital camera or a cellular phone with a camera having the image sensing device are growing. To meet these demands, the pan-focus image sensing device must have a mechanism suitable for macrophotography. However, references 1 to 6 described above have no description of this technique.
In mass-producing such image sensing apparatuses, it is difficult to accurately form an object image on the image sensing surface of the image sensing element to read an image due to variations in manufacturing dimensions of the optical system, variations in manufacturing dimensions of individual components of the holding frame of the optical system, variations of characteristics, or variations in assembling them.
To solve this problem, focus surface adjustment (to be referred to as fc adjustment hereinafter) must be done for individual image sensing apparatuses such that the position of the light-receiving surface of the image sensing element and the imaging plane can obtain the most satisfactory relationship. However, no fc adjustment mechanism suitable for such an image sensing device is described in the above prior arts. No means for implementing both the above-described mechanism suitable for macrophotography and the fc adjustment mechanism is disclosed, either.